Multi-channel WLAN transceiver with antenna diversity

ABSTRACT

An access point for use in a wireless local area network (WLAN) includes a plurality of antennas and two or more transmitters, which are adapted to transmit downlink signals on different, respective downlink frequency channels of the WLAN via at least one of the antennas. Two or more receivers are coupled to receive uplink signals via at least two of the antennas, and are adapted to receive the uplink signals simultaneously on different, respective uplink frequency channels of the WLAN.

FIELD OF THE INVENTION

[0001] The present invention relates generally to wireless communications, and specifically to methods and devices for improving the performance of wireless local area networks.

BACKGROUND OF THE INVENTION

[0002] Wireless local area networks (WLANs) are gaining in popularity, and new wireless applications are being developed. The original WLAN standards, such as “Bluetooth” and IEEE 802.11, were designed to enable communications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEE working groups have defined the 802.11a, 802.11b and 802.11g extensions to the original standard, in order to enable higher data rates. The 802.11a standard, for example, envisions data rates up to 54 Mbps over short distances in a 5 GHz band, while 802.11b defines data rates up to 22 Mbps in the 2.4 GHz band. In the context of the present patent application and in the claims, the term “802.11” is used to refer collectively to the original IEEE 802.11 standard and all its variants and extensions, unless specifically noted otherwise.

[0003] The theoretical capability of new WLAN technologies to offer enormous communication bandwidth to mobile users is severely hampered by the practical limitations of wireless communications. Indoor propagation of radio frequencies is not isotropic, because radio waves are influenced by building layout and furnishings. Therefore, even when wireless access points are carefully positioned throughout a building, some “black holes” generally remain—areas with little or no radio reception. Furthermore, 802.11 wireless links can operate at full speed only under conditions of high signal/noise ratio. Signal strength scales inversely with the distance of the mobile station from its access point, and therefore so does communication speed. A single mobile station with poor reception due to distance or radio propagation problems can slow down WLAN access for all other users in its basic service set (BSS—the group of mobile stations communicating with the same access point).

[0004] The natural response to these practical difficulties would be to distribute a greater number of access points within the area to be served. If a receiver receives signals simultaneously from two sources of similar strength on the same frequency channel, however, it is generally unable to decipher either signal. The 802.11 standard provides a mechanism for collision avoidance based on clear channel assessment (CCA), which requires a station to refrain from transmitting when it senses other transmissions on its frequency channel. In practice, this mechanism is of limited utility and can place a heavy burden on different BSSs operating on the same frequency channel.

[0005] Therefore, in high data-rate 802.11 WLANs known in the art, access points in mutual proximity must use different frequency channels. Theoretically, the 802.11b and 802.11g standards define 14 frequency channels in the 2.4 GHz band, but because of bandwidth and regulatory limitations, WLANs operating according to these standards in the United States actually have only three non-overlapping frequency channels from which to choose. As a result, in complex, indoor environments, it becomes practically impossible to distribute wireless access points closely enough to give strong signals throughout the environment without substantial overlap in the coverage areas of different access points operating on the same frequency channel.

[0006] Access points and mobile stations in a WLAN commonly use spatial diversity for increasing the capacity and reliability of their wireless links. For diversity purposes, the wireless receiver in the access point or mobile station is equipped with multiple antennas, which are spaced a certain distance apart (typically at least one half wavelength at the transmission frequency). The signals received by the different antennas vary due to environmental conditions, such as absorption and multi-path fading effects. The receiver takes advantage of these differences to compensate for degradation that may occur as the signals travel from the transmitter to the receiver, thereby improving the fidelity of received signals and increasing the effective rate at which the receiver is able to receive data.

[0007] Multi-channel receivers with multiple antennas are known in the art. For example, U.S. Pat. No. 5,842,117, whose disclosure is incorporated herein by reference, describes an antenna system for transmitter-receiver stations in a mobile radio system with at least two transmission channels and two reception channels and at least two antennas for redundant reception. Each antenna is used as a transmission antenna for at least one channel, and at least one antenna is set for redundant reception of the channel that is fed to another antenna as the transmission channel.

SUMMARY OF THE INVENTION

[0008] Embodiments of the present invention provide access points for use in a wireless local area network (WLAN), which are capable of communicating on multiple frequency channels simultaneously, unlike access points known in the art. Each such access point comprises multiple transmitters and receivers, each of which is tuned for operation on a different frequency channel, among the available channels on the WLAN. A controller determines for each of the transmitters in each access point, whether or not it should transmit at any given time. For purposes of space diversity, each access point comprises multiple antennas, and each of the receivers is coupled to receive uplink signals via at least two of the antennas, which are shared among the different channels. Thus, each multi-channel access point behaves effectively as though it were a set of several collocated single-channel access points, while avoiding the need to deploy separate antennas for each channel.

[0009] In order to provide complete coverage of the service region of the WLAN, with strong communication signals throughout the region, the access points are typically closely spaced, and their areas of coverage may substantially overlap one another. The access points communicate among themselves using a novel, low-latency protocol over a wired LAN or other communication medium connecting the access points. This protocol enables the access points to resolve the overlap of their coverage areas and to decide, for each frequency channel, which access point should transmit at any given time. Thus, for example, when a mobile station sends an uplink message asking to initiate communications on a given frequency channel, the access points receiving the uplink message arbitrate among themselves, using the protocol, to determine which one will respond. The controller in the chosen access point actuates the appropriate transmitter to respond on the given frequency channel. When required, the controller may actuate two or more of the transmitters to transmit signals simultaneously on their respective frequency channels.

[0010] There is therefore provided, in accordance with an embodiment of the present invention, an access point for use in a wireless local area network (WLAN), the access point including:

[0011] a plurality of antennas;

[0012] two or more transmitters, which are adapted to transmit downlink signals on different, respective downlink frequency channels of the WLAN via at least one of the antennas; and

[0013] two or more receivers, each of which is coupled to receive uplink signals via at least two of the antennas, and which are adapted to receive the uplink signals simultaneously on different, respective uplink frequency channels of the WLAN.

[0014] In an aspect of the present invention, the access point includes at least one radio frequency (RF) multiplexer, coupled between the at least two of the antennas and the two or more receivers, so as to convey to each of the receivers the uplink signals received via the antennas on the respective uplink frequency channels. In one embodiment, the two or more receivers include three receivers, which are tuned to operate on three respective uplink frequency channels, and the at least one multiplexer includes two triplexers, coupled respectively to two of the antennas, so as to split the uplink signals received via each of the antennas among the three receivers according to the three frequency channels.

[0015] Typically, the receivers are adapted to process the uplink signals from the at least two of the antennas simultaneously, so as to improve a fidelity of the received uplink signals based on a diversity of the antennas.

[0016] In an embodiment of the invention, the transmitters and receivers are adapted to communicate with mobile stations in the WLAN substantially in accordance with an IEEE 802.11 specification, which defines the uplink and downlink frequency channels.

[0017] In a further aspect of the invention, the access point is one of a plurality of access points in the WLAN, and includes a communication interface, coupled to a medium for communicating with the other access points, and processing and control circuitry, which is adapted to send and receive messages to and from the other access points via the communication interface, so as to make a determination of when each of the two or more transmitters should transmit the downlink signals, and which is coupled to control the two or more transmitters according to the determination. Typically, the communication medium includes a wired local area network (LAN).

[0018] The processing and control circuitry may be adapted, when one of the receivers receives an uplink signal transmitted over the WLAN by a mobile station on one of the uplink frequency channels, to send and receive the messages in accordance with an arbitration protocol carried out together with others of the access points receiving the uplink signal, so as to select one of the access points to respond to the uplink signal, and to control the two or more transmitters so that one of the transmitters returns a response to the mobile station subject to the arbitration protocol. Typically, the processing and control circuitry is adapted to control the two or more transmitters so that when the access point is selected, the response is returned to the mobile station by one of the transmitters on one of the downlink frequency channels that corresponds to the one of the uplink frequency channels on which the uplink signal was received.

[0019] Alternatively or additionally, the processing and control circuitry may be adapted to control the two or more transmitters so as to cause at least two of the transmitters to transmit the downlink signals simultaneously on the respective downlink frequency channels.

[0020] There is also provided, in accordance with an embodiment of the present invention, a system for mobile communication, including:

[0021] a communication medium; and

[0022] a plurality of access points interconnected by the medium and arranged to communicate with mobile stations in a wireless local area network (WLAN), each of at least some of the access points including:

[0023] a plurality of antennas;

[0024] two or more transmitters, which are adapted to transmit downlink signals to the mobile stations on different, respective downlink frequency channels of the WLAN via at least one of the antennas;

[0025] two or more receivers, each of which is coupled to receive uplink signals from the mobile stations via at least two of the antennas, and which are adapted to receive the uplink signals simultaneously on different, respective uplink frequency channels of the WLAN;

[0026] a communication interface, coupled to the communication medium; and

[0027] processing and control circuitry, which is adapted to send and receive messages to and from the other access points via the communication interface, so as to make a determination of when each of the two or more transmitters should transmit the downlink signals, and which is coupled to control the two or more transmitters according to the determination.

[0028] In an aspect of the present invention, the access points have respective service areas and are arranged so that at least some of the service areas substantially overlap.

[0029] There is additionally provided, in accordance with an embodiment of the present invention, a method for mobile communication, including:

[0030] arranging a plurality of access points in a wireless local area network (WLAN), at least some of the access points including two or more transmitters and two or more receivers, which are adapted to communicate with mobile stations over the WLAN on different, respective downlink and uplink frequency channels;

[0031] receiving at one of the receivers in each of one or more of the access points an uplink signal transmitted over the WLAN by one of the mobile stations on one of the uplink frequency channels;

[0032] arbitrating among the access points receiving the uplink signal so as to select one of the access points to respond to the uplink signal; and

[0033] actuating one of the transmitters in the selected one of the access points to transmit a response to the one of the mobile stations on one of the downlink frequency channels corresponding to the one of the uplink frequency channels on which the uplink signal was received.

[0034] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a block diagram that schematically illustrates a WLAN system, in accordance with an embodiment of the present invention;

[0036]FIG. 2 is a block diagram that schematically shows details of access points in a WLAN system, in accordance with an embodiment of the present invention; and

[0037]FIG. 3 is a block diagram that schematically shows further details of transceiver circuitry used in a WLAN access point, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0038]FIG. 1 is a block diagram that schematically illustrates a wireless LAN (WLAN) system 20, in accordance with an embodiment of the present invention. System 20 comprises multiple access points 22, which are configured for data communication with mobile stations 24. Each access point is capable of transmitting and receiving signals simultaneously on multiple different frequency channels that are available on the WLAN. (By contrast, in WLAN systems known in the art, each access point has a single, fixed frequency channel.) For spatial diversity purposes, as described further hereinbelow, each of access points 22 typically comprises at least two antennas 30 and 32. Mobile stations 24 typically comprise computing devices, such as desktop, portable or handheld devices.

[0039] In the exemplary embodiment described hereinbelow, it is assumed that the access points and mobile stations communicate with one another in accordance with one of the standards in the IEEE 802.11 family and observe the 802.11 medium access control (MAC) layer conventions. Details of the 802.11 MAC layer are described in ANSI/IEEE Standard 801.11 (1999 Edition), and specifically in Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, which is incorporated herein by reference. The principles of the present invention, however, are not limited to the 802.11 standards, and may likewise be applied to substantially any type of WLAN, including HiperLAN, Bluetooth and hiswan-based systems.

[0040] Access points 22 are connected to a hub 26 by a wired LAN 28, over which the access points communicate arbitration messages with one another using a novel, high-speed protocol over LAN 28 or whatever other medium is used to connect the access points, as described in the above-mentioned patent applications.

[0041] Typically, the arbitration messages are broadcast by all access points 22 that receive an uplink signal from a given mobile station, to all the other access points. Based on the arbitration messages, the access points decide which access point is to serve a given mobile station on a given frequency channel. (Usually the closest access point to the mobile station is the first to send out an arbitration message in response to a given uplink message, and is therefore chosen to respond to the uplink message. Other considerations may also be taken into account in the arbitration process, however.) The other access points meanwhile refrain from interfering.

[0042] The messages exchanged among access points 22 in this manner enable the access points to dynamically allocate frequency channels among themselves, depending on the demand for these channels by the mobile stations served by the WLAN at any given time. At different times, a given access point may transmit downlink signals on different channels, and it may even transmit on two or more channels simultaneously. Access points 22 may also use transmit power control (TPC) collaboratively to divide the WLAN into dynamic, non-interfering sub-networks. This technique is described further in U.S. patent application Ser. No. 10/285,869, filed Nov. 1, 2002, which is assigned to the assignee of the present patent application, and whose disclosure is incorporated herein by reference. The TPC-based sub-network structure allows frequency channels to be spatially reused among the access points, thus increasing the capacity of the WLAN.

[0043]FIG. 2 is a block diagram that schematically shows details of access points 22, in accordance with an embodiment of the present invention. Each access point comprises WLAN physical layer interfaces (PHY) 33, 34, 35 coupled to antennas 30 and 32. Each WLAN PHY comprises a radio transceiver, for communicating over a respective frequency channel of the WLAN. Details of these transceivers are shown below in FIG. 3. The access point is connected to LAN 28 by a LAN PHY 38, typically comprising an Ethernet interface. The LAN PHY may be used both for data transmitted to and from mobile stations 24 via LAN 28 and for low-latency control messages (such as the arbitration messages described above) among the access points. This dual use of LAN 28 is described in detail in the above-mentioned patent application entitled “LAN with Message Interleaving.” Alternatively, access points 22 may have separate interfaces for LAN 28 and for the shared media that they use for exchanging control messages.

[0044] A medium access control (MAC) processor 36 performs higher-level message processing functions. For convenience of implementation, access point 22 may comprise multiple MAC processors 36, as shown in FIG. 2, each communicating with a respective PHY 33, 34 or 35 and, thus, serving a respective frequency channel on the WLAN. One or more of the MAC processors may also be configured for transmitting and receiving control messages among the access points.

[0045] Outgoing control messages are generated by a collaboration controller 40. For example, when WLAN PHY 34 receives an uplink message on its respective frequency channel over the air from a mobile station, controller 40 generates an arbitration message, as described above. The control message is immediately transmitted via LAN PHY 38 by MAC processor 36. When the MAC processor receives an incoming control message from LAN 28, it passes the message to controller 40. Depending on the nature and contents of the control message, controller 40 may instruct MAC processor 36 and WLAN PHY 34 to transmit a downlink message on the respective frequency channel. Uplink and downlink messages to and from WLAN PHY 33 and PHY 35 are handled in like manner. Each frequency channel and its respective WLAN PHY are handled by controller 40 substantially independently of the other channels, and access point 22 may thus transmit and receive signals over the air on multiple channels simultaneously.

[0046] The functional blocks of access point 22 shown in FIG. 2 are chosen for conceptual clarity, and do not necessarily represent the physical components that might actually be used to implement the design shown here. The functional blocks shown in the figure may be combined into one or more custom integrated circuit components, or they may alternatively be broken into a larger number of custom or off-shelf components. Logical and control elements of access point 22 may comprise either hard-wired or programmable components, with appropriate software, as will be apparent to those skilled in the art.

[0047]FIG. 3 is a block diagram that schematically illustrates further details of radio circuitry used in access point 22, in accordance with an embodiment of the present invention. Each of PHYs 33, 34 and 35 comprises a radio transceiver, including a transmit circuit 64 and a receive circuit 70, which are tuned to serve the respective frequency channel of the PHY. The channels are arbitrarily identified in the figure as F1, F2 and F3. For an 802.11b WLAN, for example, the transceivers would be tuned to channel 1, channel 6 and channel 11, as provided by the 802.11b specification. Transmit circuit 64 and receive circuit 70 in each PHY are driven by an oscillator 66, which is tuned to the frequency of the channel assigned to that transceiver (F1, F2 or F3).

[0048] The radio transceivers in PHYs 33, 34 and 35 are coupled to antennas 30 and 32 by respective triplexers 58 and 60. Triplexers 58 and 60 typically comprise arrays of single-ended passive filters, as are known in the art, which combine the downlink signals from transmitters 64 with minimal loss. The triplexers similarly divide the uplink signals by frequency channel, so that the frequency channel of F1 is passed to PHY 33, F2 to PHY 34, and F3 to PHY 35. Typically, for efficient combination and separation of signals, triplexers 58 and 60 are fabricated as one or more RF hybrid components, as is known in the art. Triplexers of this sort are available from various manufacturers, such as Rojone Pty Ltd. (Sydney, Australia), Multec Communications (Rockland, Mass.) and the Radiowave Division of Unglar Inc. (Rindge, N.H.). The triplexers shown here, for use in the 2.4 GHz band of IEEE 802.11b/g, are just one example of RF multiplexers that may be used in sharing antennas among multiple WLAN channels. As another example, a six- or eight-way multiplexer could be used to serve the six to eight channels that are available in the 5 GHz band. Multiplexers for greater or smaller numbers of channels may be used, as appropriate, in these and other applications.

[0049] The internal construction of transmit circuit 64 and receive circuit 70 in each of PHYs 33, 34 and 35 is similar to that of WLAN access point transceivers known in the art (except that conventional 802.11 access points generally have only a single transceiver, which is tuned to operate on a single frequency channel). A switch 56 in each PHY performs two functions: (1) it toggles each transceiver between transmit and receive modes, typically under control of MAC processor 36; and (2) it toggles between antennas 30 and 32 for diversity purposes, typically under control of receive circuit 70. Circuit 70 downconverts and demodulates the signals, in order to generate a data stream for output to MAC processor 36. The receive circuit makes use of the antenna diversity in order to improve the fidelity of its data output, using diversity processing methods known in the art.

[0050] When a new uplink signal is received by receive circuit 70 on a given channel, collaboration controller 40 generates an arbitration message and carries out the arbitration procedure described above, to determine whether the corresponding transmit circuit 64 should respond with a downlink signal. When controller 40 has determined that a given transceiver should transmit a downlink signal on the channel in question, MAC processor 36 passes data for transmission to transmit circuit 64. Circuit 64 modulates the signal, in accordance with the modulation scheme provided by the applicable WLAN standard, and upconverts the signal to the appropriate with one another in order to coordinate their use of the available frequency channels and responses to mobile stations. LAN 28 typically comprises an Ethernet LAN, such as a 100BASE-TX LAN, which may also be used as a distribution system (DS) for carrying data to and from mobile stations 24. Alternatively or additionally, the access points may communicate with one another over substantially any suitable high-speed communication medium, including wire, fiberoptics, or even free-space optical or radio communications (in an allowed frequency band that does not interfere with WLAN operation). Methods and media that can be used for communication among the access points are described, for example, in U.S. patent application Ser. No. 10/214,271, filed Aug. 7, 2002, in U.S. patent application Ser. No. 10/272,686, filed Oct. 17, 2002, and in a further U.S. patent application, entitled “LAN with Message Interleaving,” filed Jan. 22, 2003. These three applications are assigned to the assignee of the present patent application, and their disclosures are incorporated herein by reference.

[0051] Access points 22 in system 20 are typically closely spaced, so that radio waves in a given frequency channel may reach mobile station 24 from multiple access points simultaneously, and radio messages transmitted by the mobile station may be received at multiple access points. In WLAN systems known in the art, under these circumstances, mobile station 24 would receive downlink messages from two or more of the access points, which would probably result in inability of the mobile station to communicate with any of the access points. In embodiments of the present invention, the access points collaborate to resolve this conflict by exchanging radio frequency channel. Switch 56 couples the downlink signal to one or both of triplexers 58 and 60, for transmission via antenna 30 and/or 32. Typically, switch 56 selects one of the antennas to use in transmitting the downlink signal, depending on which of the antennas received the preceding uplink signal with better quality. Alternatively, switch 56 may simply select the same antenna for transmission at all times.

[0052] Although in the embodiment described above, access points 22 have three RF channels and two antennas, the principles of the present invention may similarly be applied to access points with a greater number of antennas, serving substantially any number (two or more) of RF channels. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. 

1. An access point for use in a wireless local area network (WLAN), the access point comprising: a plurality of antennas; two or more transmitters, which are adapted to transmit downlink signals on different, respective downlink frequency channels of the WLAN via at least one of the antennas; and two or more receivers, each of which is coupled to receive uplink signals via at least two of the antennas, and which are adapted to receive the uplink signals simultaneously on different, respective uplink frequency channels of the WLAN.
 2. The access point according to claim 1, and comprising at least one radio frequency (RF) multiplexer, coupled between the at least two of the antennas and the two or more receivers, so as to convey to each of the receivers the uplink signals received via the antennas on the respective uplink frequency channels.
 3. The access point according to claim 2, wherein the two or more receivers comprise three receivers, which are tuned to operate on three respective uplink frequency channels, and wherein the at least one multiplexer comprises two triplexers, coupled respectively to two of the antennas, so as to split the uplink signals received via each of the antennas among the three receivers according to the three frequency channels.
 4. The access point according to claim 1, wherein the receivers are adapted to process the uplink signals from the at least two of the antennas simultaneously, so as to improve a fidelity of the received uplink signals based on a diversity of the antennas.
 5. The access point according to claim 1, wherein the transmitters and receivers are adapted to communicate with mobile stations in the WLAN substantially in accordance with an IEEE 802.11 specification, which defines the uplink and downlink frequency channels.
 6. The access point according to claim 1, wherein the access point is one of a plurality of access points in the WLAN, and comprises: a communication interface, coupled to a medium for communicating with the other access points; and processing and control circuitry, which is adapted to send and receive messages to and from the other access points via the communication interface, so as to make a determination of when each of the two or more transmitters should transmit the downlink signals, and which is coupled to control the two or more transmitters according to the determination.
 7. The access point according to claim 6, wherein the communication medium comprises a wired local area network (LAN).
 8. The access point according to claim 6, wherein the processing and control circuitry is adapted, when one of the receivers receives an uplink signal transmitted over the WLAN by a mobile station on one of the uplink frequency channels, to send and receive the messages in accordance with an arbitration protocol carried out together with others of the access points receiving the uplink signal, so as to select one of the access points to respond to the uplink signal, and to control the two or more transmitters so that one of the transmitters returns a response to the mobile station subject to the arbitration protocol.
 9. The access point according to claim 8, wherein the processing and control circuitry is adapted to control the two or more transmitters so that when the access point is selected, the response is returned to the mobile station by one of the transmitters on one of the downlink frequency channels that corresponds to the one of the uplink frequency channels on which the uplink signal was received.
 10. The access point according to claim 6, wherein the processing and control circuitry is adapted to control the two or more transmitters so as to cause at least two of the transmitters to transmit the downlink signals simultaneously on the respective downlink frequency channels.
 11. A system for mobile communication, comprising: a communication medium; and a plurality of access points interconnected by the medium and arranged to communicate with mobile stations in a wireless local area network (WLAN), each of at least some of the access points comprising: a plurality of antennas; two or more transmitters, which are adapted to transmit downlink signals to the mobile stations on different, respective downlink frequency channels of the WLAN via at least one of the antennas; two or more receivers, each of which is coupled to receive uplink signals from the mobile stations via at least two of the antennas, and which are adapted to receive the uplink signals simultaneously on different, respective uplink frequency channels of the WLAN; a communication interface, coupled to the communication medium; and processing and control circuitry, which is adapted to send and receive messages to and from the other access points via the communication interface, so as to make a determination of when each of the two or more transmitters should transmit the downlink signals, and which is coupled to control the two or more transmitters according to the determination.
 12. The system according to claim 11, wherein the access points have respective service areas and are arranged so that at least some of the service areas substantially overlap.
 13. The system according to claim 11, wherein each of the at least some of the access points comprises at least one radio frequency (RF) multiplexer, coupled between the at least two of the antennas and the two or more receivers, so as to convey to each of the receivers the uplink signals received via the antennas on the respective uplink frequency channels.
 14. The system according to claim 11, wherein the receivers in each of the at least some of the access points are adapted to process the uplink signals from the at least two of the antennas simultaneously, so as to improve a fidelity of the received uplink signals based on a diversity of the antennas.
 15. The system according to claim 11, wherein the transmitters and receivers are adapted to communicate with mobile stations in the WLAN substantially in accordance with an IEEE 802.11 specification, which defines the uplink and downlink frequency channels.
 16. The system according to claim 11, wherein the communication medium comprises a wired local area network (LAN).
 17. The system according to claim 11, wherein the processing and control circuitry is adapted, when one of the receivers receives an uplink signal transmitted over the WLAN by a mobile station on one of the uplink frequency channels, to send and receive the messages in accordance with an arbitration protocol carried out together with others of the access points receiving the uplink signal, so as to select one of the access points to respond to the uplink signal, and to control the two or more transmitters so that one of the transmitters returns a response to the mobile station subject to the arbitration protocol.
 18. The system according to claim 11, wherein the processing and control circuitry is adapted to control the two or more transmitters so as to cause at least two of the transmitters in one of the access points to transmit the downlink signals simultaneously on the respective downlink frequency channels.
 19. A method for mobile communication, comprising: arranging a plurality of access points in a wireless local area network (WLAN), at least some of the access points comprising two or more transmitters and two or more receivers, which are adapted to communicate with mobile stations over the WLAN on different, respective downlink and uplink frequency channels; receiving at one of the receivers in each of one or more of the access points an uplink signal transmitted over the WLAN by one of the mobile stations on one of the uplink frequency channels; arbitrating among the access points receiving the uplink signal so as to select one of the access points to respond to the uplink signal; and actuating one of the transmitters in the selected one of the access points to transmit a response to the one of the mobile stations on one of the downlink frequency channels corresponding to the one of the uplink frequency channels on which the uplink signal was received.
 20. The method according to claim 19, wherein the access points have respective service areas, and wherein arranging the plurality of access points comprises arranging the access points so that at least some of the service areas substantially overlap.
 21. The method according to claim 19, wherein receiving the uplink signal comprises receiving the signal via at least two antennas at each of the one or more of the access points, wherein the antennas are coupled to the two or more receivers so that uplink signals are split among the receivers according to the respective uplink frequency channels.
 22. The method according to claim 21, wherein receiving the uplink signal comprises processing the uplink signal from the at least two antennas simultaneously, so as to improve a fidelity of the received uplink signals based on a diversity of the antennas.
 23. The method according to claim 19, wherein arranging the plurality of access points comprises configuring the transmitters and receivers so as to communicate with the mobile stations in the WLAN substantially in accordance with an IEEE 802.11 specification, which defines the uplink and downlink frequency channels.
 24. The method according to claim 19, wherein arbitrating among the access points comprises sending and receiving messages among access points via a communication medium, so as to select the one of the access points to respond to the uplink signal. 